The present invention relates to a vehicle-mounted telecommunication system in which master and terminal stations are coupled by cable conductors. In one application, the master station is a radio tranceiver located in a trunk room of the vehicle and the terminal station is a telephone handset located in the passenger compartment.
In a prior art vehicle-mounted system, the terminal station comprises a first differential amplifier circuit that receives signals from the master station over a pair of cable conductors, one of which is grounded at the master station to allow passage of load current from the terminal station. A second differential amplifier circuit is provided having a first input terminal connected to the ground conductor and a second input terminal connected to one terminal of a signal source the other terminal of which is connected to the ground conductor. This type of prior art system simplifies the interface of the master station, but suffers from noise generated by a large amount of load current passing through the ground conductor when the terminal station drives a heavy load such as loudspeaker.
The noise problem has been solved by a prior art system which employs a first pair of differential amplifiers on a transmission circuit from master station to terminal station and a second part of differential amplifiers on a transmission circuit from terminal station to master station. The receiving end of each transmission circuit is formed by a differential amplifier having a balancing resistor network to minimize the effect of noise generated in a ground conductor on the transmission circuits.
However, the noise-free terminal station is designed exclusively for use with master stations with the noise-free configuration. Difficulty thus arises when it is desired to connect a noise-free terminal station to an existing master station having no noise-free arrangement.